1. Field of the Invention
This invention relates to devices for magnetically erasing the magnetically recorded information from record bearing media.
2. Description of the Prior Art
The conventional erasing devices in/for magnetic recording and/or reproducing apparatus for recording information, such as video signals on record bearing media such as magnetic sheets, and/or reproducing the recorded information may be divided broadly into the following categories:
(a) Information is erased from all the recording tracks at once by a bulk eraser;
(b) Besides the recording and/or reproducing head, an erasing head is provided, and the information is erased from track to track by an A.C. or D.C. erase signal; and
(c) The recording and/or reproducing head also serves for erasing purposes, and a gradually damping direct current with its polarity changed in a prescribed period is applied thereto when each recording track is erased.
Of these, (a) is incapable of erasing a desired track alone. In (b), the use of the erasing head in addition to the recording and/or reproducing head calls for an increase in the complexity of the head mechanism. (c) takes a considerably long time to completely remove the residual strain because erasing is effected by the D.C. field that is gradually damping and which changes its polarity in a prescribed period.
To eliminate the above-described problem, according to one of the preceding patent applications assigned to the assignee of the present invention, there has already been proposed an erasing device in which the erase signal for causing the magnetic head to generate a magnetic field which has a form which includes a wave of constant amplitude for a time period T.sub.E1 (see FIG. 1) followed by a damping wave for a time period T.sub.E2 with their frequencies being low outside the frequency band of the recording or reproducing signals, whereby the information can be erased in any desired recording track and at an over-widened width of the recording track with improved efficiency. This is disclosed in Japanese Patent Application No. SHO 57-219659 (corresponding to U.S. patent application Ser. No. 560,413 filed Dec. 12, 1983).
According to another preceding patent application assigned to the assignee of the present invention, there has already been proposed an erasing device in which the time necessary to remove the residual strain is shortened either by increasing the frequency of the aforesaid damping wave from that of the constant-amplitude wave, or by lowering the relative speed of the record bearing medium to the magnetic head in the time T.sub.E2 from that in the time T.sub.E1. This is disclosed in Japanese Patent Application No. Sho 58-15821 (corresponding to U.S. patent application Ser. No. 576,034 filed Feb. 1, 1984).
In the above-described erasing devices according to the patent applications assigned to the assignee of the present invention, the erasing devices use the alternating erase signal. Upon further investigation of the interrelationship between the relative rotation of the magnetic head and the record bearing medium and the magnetic field produced on the magnetic head by the erase signal, it has been found that when the frequency of the aforsaid relative rotation and the frequency of the aforesaid erase signal have a certain relation, a number of nodes appear on the erasing field fixed at positions on the track of the record bearing medium for a plurality of relative rotations of the medium and the head. The point is that sufficient erasing cannot be achieved at these nodes, and there is room for still more improvement.
For example, FIG. 2 illustrates the manner in which an erasing magnetic field is formed along the selected concentric track of the disc-shaped magnetic sheet which is erased by applying the erase signal of FIG. 1 to the magnetic head, while the magnetic sheet is being rotated. In FIG. 2, H.sub.E1 and H.sub.E2 identify two waves of the erasing field arbitrarily chosen from those due to the erase signal of FIG. 1. When the frequency f.sub.E of the erase signal and the frequency f.sub.V of rotation of the magnetic sheet lie in the following relationship: EQU f.sub.E =(k/2)f.sub.V (where k is a positive integer)
the erasing field has nodes always at points P1, P2, . . . , P3 and P4 along the entire length of any one of the aforesaid recording tracks for every rotation of the magnetic sheet. It is at these points that no erasing occurs.
It is therefore an object of the present invention to provide an erasing device in which an erase signal, which varies periodically, is applied to a magnetic head to form a magnetic field by which the information on a record bearing medium is erased during the relative rotation of the medium and the entire track on the medium is properly and sufficiently erased in spite of the occurrence of the nodes of the erasing magnetic field due to the periodic change of the erase signal.
Another object of the invention is to provide an erasing device with a simple circuit structure which increases the frequency accuracy of an erase signal, whereby the residual noise is lessened, and high erasing performance is obtained.
Still another object of the invention is to provide an erasing device very well suited for instruments dealing with TV signals.
Other objects, aspects and features of this invention will become apparent from the following description of embodiments thereof by reference to the accompanying drawings.